1. Field of the Invention
The present invention relates to a substrate structure, a light emitting device (LED) unit having such substrate structure, and a light source module utilizing such LED unit. Particularly, the present invention relates to a LED substrate structure, a LED unit and a lighting module having the same.
2. Description of Related Art
LEDs can be disposed on a printed circuit board (PCB) through surface-mount technique (SMT) as a surface-mount device. Depending on the illumination style/arrangement of the LED die, the surface mounted LEDs can be further classified into a side-view type and a top-view type.
In the conventional LED unit assembling process, at least one LED die is mounted on a substrate and then encapsulated by a packaging material to form a LED unit. In the side-view type LED unit, the LED unit is mounted side-way on the PCB in such a way that a lateral surface adjacent to a rear metal portion of the LED unit may establish contact with a mounting surface of the PCB, so that the rear metal portion is arranged adjacent to a solder pad on the PCB. Particularly for this arrangement, soldering material is deposited at the junction where the rear metal portion meets the solder pad of the PCB (hereinafter referred to as “contact border”). The melted soldering material would vertically “climb onto” the real metal portion, thus fixing the LED unit side-way on the PCB. To enhance the fixing effect of the LED unit on the PCB, i.e. soldering ability, the rear metal portion of a conventional side-view type LED unit often takes a triangular or cuplike shape, where the width of the metal portion is greatest proximate the contact border and narrowest toward the center portion thereof. As the singulation process for each individual LED unit is carried out, burrs on the metal portion along the cutting line often occur as a result. The burr along the edge of the metal portion (where the width thereof is the widest) creates a long and unsmooth segment that makes it difficult for soldering. In other words, the burrs formed on the metal portion of the LED unit along the cutting path may reduce the soldering material's adhesion ability. Particularly, under common low temperature soldering process conditions in which soldering material still retain high viscosity and thus exhibits poor flowability/wettability, poor soldering may occur more often. As a result, production yield would be negatively affected.
On the other hand, the LED die is fixed on a die-bonding area of the substrate through the coverage of the packaging material. However, the packaging material is often made of plastics while the die-bonding area is made of metal. Due to the difference in material characteristics between the plastic packaging material and the metallic die-bonding area of the substrate, the contact interface therebetween may exhibit inadequate bonding strength. As a result, the packaging material may spill out through the interface or the soldering material may undesirably permeate into the LED packaging material. In addition, moisture from the surrounding environment may enter into the LED through the plastic-metal boding interface to contaminate the LED die. As a result, the reliability and the stability of the traditional LED unit suffer.